4-Alkylamino-3-penten-2-ones (R-APO), R = H, CH3, C2H5, CH(CH3)2, and C(CH3)3, were synthesized by amination of acetylacetone and the products were characterized by 1HNMR spectroscopy. The intramolecular hydrogen bonding (IHB) of R-APO molecules is studied, using 1HNMR and density functional theory calculations, also the effects of substitutions are investigated. According to the calculation results, the number of stable theoretical conformations of ethyl, isopropyl, methyl, and t-butyl substitutions is 2, 1, 1 and 1, respectively. All calculations are performed at the B3LYP/6–311++G(d,p) level. To understand the substitution effects on the nature of IHB and the electronic structure of the chelated ring system, the topological parameters, 1H NMR chemical shifts, geometrical parameters, natural bond orders, vibrational frequencies, and the natural charges over atoms involved in the chelated ring of 4-amino-3-penten-2-one (APO) and its derivatives were calculated. The Wiberg bond orders and the natural charges over atoms involved in the chelated ring have been computed by using the natural bond orbital (NBO) analysis. Based on the experimental 1HNMR and theoretical results, the strength of the intramolecular hydrogen bond in APOs is in the following order: t-Bu > ISO∼ Et ∼ Met > H. Several correlations between geometrical and topological parameters, vibrational wavenumbers, bond orders, and natural charges with the IHB strength and 1H NMR chemical shift are obtained.

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